Particulate filter media having a gradient of removal ratings

ABSTRACT

A particulate filter bed in which the particles define a generally uniform gradient of removal ratings of decreasing coarseness from top to bottom, and a method of producing the filter particles therefor are provided. The bed comprises a multiplicity of particles of uniformly decreasing size and increasing specific gravity from top to bottom. The particles have cores of approximately equal size and specific gravity and outer coatings of varying thicknesses, but equal specific gravity. The specific gravity of the outer coating is substantially less than the specific gravity of the core, so that the overall specific gravity of the larger particles is less than that of the smaller particles. The method comprises encapsulating the higher specific gravity core material with lesser specific gravity material by spray coating or the like.

United States Patent Lerner et a1.

[54] PARTICULATE FILTER MEDIA HAVING A GRADIENT OF REMOVAL RATINGS [72]Inventors: Marc Lerner, Swan Lake, N.Y. 12783; Giora Erlich, 8 SolandRoad, Monsey,N.Y.

22 Filed: Aug. 30, 1971 2| Appl. No.: 176,182

152] US. Cl ..2l0/290, 210/504 [51] Int. Cl. ..B01d 23/10 [58]Fieldoi'Search ..ll7/100B,l00D,l00M,

[45) Dec. 5, 1972 3,483,012 12/1969 Young 1 17/100 8 PrimaryExaminer-Samih N. Zaharna ArrorneyLawrence S. Lawrence [57] ABSTRACT Aparticulate filter bed in which the particles define a generally uniformgradient of removal ratings of decreasing coarseness from top to bottom,and a method of producing the filter particles therefor are provided.The bed comprises a multiplicity of particles of uniformly decreasingsize and increasing specific gravity from top to bottom. The particleshave cores of approximately equal size and specific gravity and outercoatings of varying thicknesses, but equal specific gravity. Thespecific gravity of the outer coating is substantially less than thespecific gravity of the core, so that the overall specific gravity ofthe larger particles is less than that of the smaller particles. Themethod comprises encapsulating the higher specific gravity core materialwith lesser specific gravity material by spray coating or the like.

7 Claims, 3 Drawing Figures PKTENTEDBH: .5 I972 INVENTORS MARC LERNERGIORA ERLICH AT TORNE Y PARTICULATE FILTER MEDIA HAVING A GRADIENT OFREMOVAL RATINGS In filter units comprising beds of particulate filterirregular shape, so that the bed has a multiplicity of tortuous flowpassages extending between the particles. The size of the particles andtheir bulk density determine the size of the passages and thus thefiltration rating of the bed, the smaller the particles the finer thefilter. Due to the generally random distribution of the particles withina bed and their uniform size distribution over a generally controlledrange, the size of the flow passages in the bed varies between somewhatpredeterminedlimits. It is this variance in the flow passage size whichpermits the bed to function as a relatively efficient filter.

A contaminant particle in the influent stream enters a flow passage inthe bed and travels downwardly therein until its movement is obstructedby a contriction in the flow passage. If the contaminant particle issmaller than the smallest constriction in the flow passage it enters, itwill obviously pass through the filter bed. But bed type filters areusually employed in recirculating fluid systems, so that a contaminantparticle not trapped by the filter on one pass may become trapped on asubsequent pass. This is true because the particle may enter a passagehaving a smaller contriction on a subsequent pass and more importantlybecause the filter rating becomes progressively finer as contaminantparticles are trapped within the bed. The entrained contaminantparticles serve as additional particulate filter media of reduced sizeand thus reduce the filtration rating of the bed.

While this phenomonum is useful in providing and maintaining high degreeof filtration, it also creates several drawbacks. The entrainedcontaminant particles retained near the top of the bed rapidly increasethe differential pressure across the filter and thereby reduce the flowcapacity of the entire system. To remedy this condition the height offilter beds has been increased and frequent backwashing to discharge theentrained particles has been required. However, this remedy is onlyeffective where there is a random distribution of contaminant particlesizes within the range of filter passage sizes so that thefmajority ofthe particles that enter the bed are trapped at some point along itsheight. If the contaminant particles in the system are large withrespect to the size of the flow passages in the bed, the life of thefilter will be relatively short regardless of the height of the bed,since the upper portion of the bed will retain most of the contaminants.The major portion of the bed is still clean when the differentialpressure becomes high enough to require backwashing. Thus, the fullcapacity of the filter is not utilized. This is particularly true whenthe contaminants are relatively large flat objects such as leavesnormally encountered in swimming pool water circulation systems. Theleaves or similar debris are retained on the top of the bed andaffectively block normal flow through the bed.

Stratified or multilayered particulate filtration media have beenproposed as a means for overcoming these dificiencies. Such mediaprovide a coarse prefiltration layer to remove large contaminantparticles without premature plugging and one or more fine downstreammedium, the filter particles are usually spherical or of 5 filtrationlayers to remove smaller contaminant particles which are not retained bythe prefilter. in this manner it was thought possible that the fullcapacity of the filter medium could be substantially utilized beforebackwashing was required.

in U.S. Pat. No. 293,745 to Hyatt a stratified filter bed is disclosedcomprising multiple horizontally disposed layers of particulate filtermaterial of different size and specific gravity in which the particlesin each layer are larger and of lesser specific gravity than theparticles in the layer below. Since there is little tendency for largeparticles of lesser specific gravity to sink through the bed, the filterlayers are maintained.

Although the advantages of providing a filter medium having layers ofdifferent filtration ratings were recognized almost a century ago byHyatt, this principle of filtration has never been put into widespreadpractice. The reason for this seems to have been the difficulty and costinvolved in obtaining the required particles of the proper size andspecific gravity. This is particularly true where more than two layersof varying size are desired. Hyatt discloses several different types ofmaterial which can be employed in his filter, but the specific gravitiesof the materials disclosed are not sufficiently different to avoidmixing with the particles of the adjacent layers. This is not a problemif when the filter is initially set up care is used in placing thevarious layers in a filter tank. However, when the bed eventuallybecomes contaminated and requires backwashing, the reverse flow causesextensive mixing of the particles in the several layers. Since thespecific gravity of particles involved are fairly close to one anotherthe return to the normal filtration mode does not necessarily force theparticles back to their assigned layers. The bed can thereby become auniform mixture of particles with a broad range of sizes. This seriouslydecreases the removal efficiency of the filter, since the bed in suchcondition possesses both coarse and fine flow passages in paralleljuxtaposition.

In accordance with the present invention a particulate filter bed havingparticles defining a uniform gradient of decreasing removal ratingsranging from coarser on top to finer on the bottom is disclosed, whichovercomes the deficiencies found in the above described Stratifiedfiltration medium, and which greatly improves filtration effeciency bymaximizing the number of different size passages through which theinfiuent fluid must pass. This permits efficient utilization of thefilter medium in fluid systems having a greater range of contaminantparticle sizes than could be accomodated by prior filter beds.

in addition, the present invention provides a filter bed in which therelationship between filter particle sizes and specific gravities issuch that no matter how the particles are placed in a filter tank orpositioned therein after a backwashing cycle, when they are subjected tonormal flow, the combined force of gravity and flow will arrange theparticles in a generally uniform gradient of decreasing particle sizesfrom top to bottom.

A method of forming the filter media particles of the invention is alsoprovided, whereby the size and specific gravity of the particles can beset to the desired values. This permits efficient use of many differenttypes of material suitable for filtration purposes but heretofore notadaptable to multilayered beds.

The particulate filter medium bed of the invention comprises amultiplicity of particles of generally decreasing size and increasingspecific gravity from top to bottom, said particles having cores ofapproximately equal size and specific gravity and outer coatings ofvarying thicknesses but approximately equal specific gravity, thespecific gravity of the outer coatings being substantially less than thespecific gravity of the core, so that the overall specific gravity ofthe larger particles is less than that of the smaller particles.

By providing particles having equally sized cores and outer coatingthicknesses which are randomly distributed over a relatively large rangethe filter removal rating gradient has an equally large range. Generallyparticles having outside diameters in the range from about 0.005 to0.125 inches or larger provide a gradient of removal ratings which issuitable for use in most recirculating fluid systems. The randomlydistributed coating thicknesses prevent formation of definitive stratawithin the bed of particulate material because of the fact that very fewparticles will have exactly the same size. Since particles of differentsize have different specific gravity, they tend to arrange themselves injuxtapositions of decreasing size from top to bottom. The filter mediaof the invention is therefore more efficient and more adaptable for usein systems of varying particle contaminant size than simply stratifiedor multilayered filter media.

In general, the method of forming the particles of filter mediacomprises coating the higher specific gravity core particles by passingthem through a chamber into which the lower specific gravity coatingmaterial is introduced. The coating material is adapted to adhere to thecore and to itself so that the greater the distance the core materialtravels within the chamber of coating material, the thicker will be thecoating, the larger the resultant particle and the lesser the overallspecific gravity. To obtain a random distribution of coating thicknessesupon the core material, the core particles are introduced into thechamber, and the chamber designed in a manner such that the distance thecore material travels within the coating chamber also randomly varies.Since the coating thickness is a function of the distance traveled bythe core material within the coating chamber, it is apparent that thesmallest particles in the filter media may be uncoated core particles.Accordingly, it is preferable to employ a coating chamber that willpermit some particles to pass through without being coated.

After coating, the batch of randomly sized particles can be simplyplaced within a filter tank and subjected to flow in the normal top tobottom filter flow direction. This initial flow together with the forceof gravity tends to arrange the particles according to their specificgravity, the smaller higher specific gravity particles sinking to thebottom while the larger lower specific gravity particles rise to the topof the bed.

In order to obtain the desired differential in specific gravity betweenthe particles of varying sizes it is important that the core materialhave a specific gravity which is at least twice that of the coatingmaterial. This permits a gradual reduction in overall specific gravityas the outer coating increases in thickness, and results in the desireduniform stacking of larger particles upon smaller particles. If thedifference in specific gravity between the core material and the outercoating material is less than the desired amount there is a tendency forparticles of different sizes to co-mingle rather than to build upuniformly to form the desired filter gradient.

The core material particles in a given bed preferably have approximatelyequal sizes to ensure proper reduction of specific gravity in accordancewith the increase in outer coating thickness. The size of the coreparticles fixes the minimum passage size in the bed and is selectedtaking this factor into consideration. Generally, particles in the rangeof about 0.005 to about 0.050 inches are suitable. These particles canbe of uniform or irregular shape, but irregularly shaped particles arepreferred since they are more readily coated with the lesser specificgravity outer material.

Any relatively hard material having a specific gravity of greater thanabout 2.5 can be employed as the core material. Materials such as silicasand, crushed limestone and the like are quite suitable. Metalicmaterials such as steel, stainless steel, lead, copper and others canalso be employed.

The outer coating must be formed from a material which is readilyappliable and adherable to the core material. Thermoplastic resins areparticularly well suited for this purpose. Materials such aspolyethylene, polypropylene, polyvinyl chloride, nylon, ABS, Teflon andthe like are satisfactory. These materials can be readily applied to thecore material in their molten state and upon hardening form a toughinert surface which can be used in most fluid circulating systemsincluding those of a relatively corrosive nature, without the danger ofchemical attack or deterioration.

Several different types of coating procedures are available. Forexample, heated core particles such as sand can be passed through afluidized bed of aggitated powdered thermoplastic resin. The plasticwill melt and adhere to each particle in varying degrees, so that afairly random distribution of particle sizes will be achieved.

Another coating method consists of wetting the core particles beforepassing them through the powdered resin to facilitate adhesion betweenthe cores and the powder. The wetting agent is preferably thermoplasticsolvent, but can also be water. The powder coated particles are thenheated to the thermoplastic melting point and cooled to obtain theproper hard coating.

Still another coating method is accomplished by passing cooled coreparticles through a molten bath of thermoplastic resin. The particlescan be introduced at various depths in the molten bath.

A further coating method comprises electrostatically charging the coreparticles and projecting the charged particles through a fluidized bedof oppositely charged powdered thermoplastic resin. The powder willadhere to and coat the cores and can be permanently bonded in place byheating the coated particles to the thermoplastic melting point and thencooling. The thickness of the coating depends on the distance traveledby the core within the bed.

As an alternative the charged powder as well as the core particles canboth be introduced in a stream into a coating chamber where they comeinto contact with each other. Naturally, this coating procedure iseffective only if the core particles are capable of accepting anelectrostatic charge. Accordingly, metallic and metallic oxide particlesand other material containing a portion of metals or metal oxides arepreferred.

A preferred coating method comprises passing the core particles at arelatively high velocity through a chamber into which moltenthermoplastic resin is sprayed. The particles which successfully crossthe entire chamber become coated with a thicker resin coating than thoseparticles which merely fall as they enter the chamber. Accordingly, agenerally random distribution of coated particle sizes can be readilyobtained.

The filter media of the invention and the method for producing it isfurther explained with reference to the drawings in which:

FIG. 1 is a cross-sectional view of a filter assembly containing theparticulate filter media.

FIG. 2 is a cross-sectional view of several coated particles and theirrelationship to one another.

FIG. 3 is a diagrammatic view of the preferred coating means foreffectuating the method of forming the filter media particles of theinvention.

The filter assembly shown in FIG. 1 comprises a tanklike housing 1having an inlet 2 and an outlet 3, and a bed 4 of particulate filtermedia 5 disposed in the tank in the line of flow from the inlet to theoutlet. A perforated plate 6 or other suitable means disposed in thetank beneath bed 4 prevents the particles 5 from being carried out byeffluent filter flow.

The particles 5 are of randomly varying size ranging from about 0.005 to0.125 inches in diameter and are positioned in the tank in generallydecreasing order from top to bottom so that filter bed 4 comprises auniform gradient of decreasingly sized filter flow passages ranging fromcoarser on top to finer on the bottom. This is accomplished by employingparticles the specific gravities of which are inversely proportional totheir size; the smaller particles having greater specific gravities thanthe larger particles. The specific gravity differential betweenparticles of different size effectuates through the force of gravity agenerally uniform stacking effect of larger particles upon smallerparticles. There are, of course, in any random distribution of particlesizes groups of particles having the same size and these particles willtend to be arranged at the same vertical levelv However, the number ofequal size particles is preferably small, so that, the formation ofdefinitive layers or strata particularly in the central portions of thebed 4 is kept to a minimum.

The gradient of decreasingly sized flow passages through which the flowproceeds within bed 4 greatly increases the contaminant removalefficiency over conventional filter beds having co-mingled particles ofvarying size or stratified layers of varied size particles. Since theuppermost particles are largest, there is little tendency for the top ofthe bed to become prematurely plugged with large contaminant particles.Likewise, small contaminant particles which would ordinarily passthrough a filter bed are entrained in the lower portions of the bed.

The cleanability of the filter bed of the invention through backwashingis also superior to prior particulate filters. When the bed is subjectedto reverse backwash flow the flow passages through which the entrainedcontaminant particles must pass to be discharged increase in size as thecontaminant particles rise in the bed, thus easily releasing them.

As shown in FIG. 2 the particles 5 of the filter bed 4 comprise an innercore 10 of silica sand and an outer coating 12 of thermoplastic resinencapsulating the core. All of the cores 10 are of approximately equalsize, while the outer coatings are of varying thicknesses. In thismanner, since the specific gravity of the thermoplastic coating isapproximately one-half that of the sand, the overall specific gravity ofeach particle 5 is inversely proportional to its size, the largerparticles having a lesser specific gravity than the smaller particles.

As an alternative to encapsulating every filter particle in bed 4, thesmallest particles can comprise merely uncoated cores 10. Since theseparticles will necessarily have the highest specific gravity in the bed,they will sink to the lower most portion thereof. Similarly, the largestparticles in the bed can be plastic or other material of approximatelythe same specific gravity with no core. Although thermoplastic materialis preferred for the outer coating because of its ability to be appliedto the core, where no core is employed both thermoplastic andthermo-setting materials are suitable.

The particles 5 can be satisfactorily formed in accordance with theinvention by utilizing the apparatus illustrated in FIG. 3. Corematerial 10 as described above is contained in hopper 20. The hopper iscon treated at its lower end to a nozzle 23 by means of a venturi 28.Nozzle 23 is disposed through the wall 29 of a chamber 21 in whichparticles 5 are coated. A source of high pressure air 26 connected toventuri 28 is adapted to provide sufficient air velocity to projectparticles S from the nozzle 23 into chamber 21.

Thermoplastic coating material 31 in its liquid state is stored insealed reservoir 22. A plurality of spray nozzles 24 disposed at the topof chamber 21 communicate with reservoir 22 by means of manifold 25. Agas burner or the like 30 disposed beneath reservoir 22 is provided toheat the thermoplastic material 31 and thereby maintain its liquidity.High pressure air introduced by means of line 27 pressurizes thereservoir to provide sufficient motive force for the thermoplasticmaterial to be sprayed into chamber 21 through nozzles 24.

In operation, large quantities of particles 10 are projected intochamber 21 simultaneously with the spraying therein of liquidthermoplastic material. As the particles 10 are subjected to thethermoplastic spray, they become coated therewith. The core particlesare maintained at a relatively cool temperature, so that the coatingmaterial solidifies almost immediately upon contact. The thickness ofthe coating built up on the cores is dependent upon the length of timethey are in contact with the spray. Those particles which are projectedacross the entire chamber will generally have thicker outer coatingsthan those particles which fall immediately upon entering the chamber.

Although the core particles 10 are all approximately the same size, theyare preferably of irregular shape. It is this factor together with theinterference of one particle with another that determines at what pointwithin chamber 21 a particle will fall out of contact with thethermoplastic spray. The falling point of a particle within the chamberis generally random. Accordingly, the overall thicknesses of thethermoplastic coatings on the cores are randomly distributed.

As the coated particles fall they are discharged from the chamber viainclined lower surface 32 to container 25. The particulate filter mediain container can be placed directly into a filter housing as shown inFIG. 1. Since the larger particles have a lesser specific gravity thanthe smaller particles, the force of gravity and the initial filter flowtends to arrange the particles in a gradient of decreasing size from topto bottom.

Although the invention has been described primarily with regard tospecific embodiments thereof, other embodiments within the scope of theinvention will be apparent to those skilled in the art.

We claim:

1. In a filter assembly having a housing with an inlet and an outlet andparticulate filter media intermediate said inlet and outlet, theimprovement comprising said particulate filter media bed defining agenerally uniform gradient of decreasing removal ratings ranging fromcoarser on top to finer on the bottom and including a multiplicity ofparticles of generally decreasing size and increasing specific gravityfrom top to bottom; said particles having cores of approximately equalsize and specific gravity and outer coatings of varying thicknesses butapproximately equal specific gravity, the specific gravity of the outercoating being substantially less than the specific gravity of the core,so that the overall specific gravity of the larger particles is 'lessthan that of the smaller particlesw 2. Particulate filter mediaaccording to claim I, in which the specific gravity of the core materialis at least twice the specific gravity of the outer coating material.

3. Particulate filter media according to claim 1, in which the outercoating.material is thermoplastic resin.

4. Particulate filter media according to claim 1, in which the corematerial is silica sand.

5. Particulate filter media according to claim 1, in which the cores areof irregular shape.

6. Particulate filter media according to claim 1, in which the coreshave diameters in the range of about 0.005 inches to about 0.050 inches;and the overall particle sizes are randomly distributed over the rangeof about 0.005 inches to about 0. inches.

7. A filter assembly comprising:

a housing having an inlet at the top and an outlet at the bottom; and

a bed of particulate filter media disposed in the housing in the line offlow from the inlet to the outlet; the particles of said bed defining agenerally uniform gradient of decreasing removal ratings ranging fromcoarser on top to finer on the bottom, said bed comprising amultiplicity of particles of generally decreasing size and increasingspecific gravity from top to bottom; said particles having cores ofapproximately equal size and specific gravity and outer coatings ofvarying thickness but approximately equal specific gravity, the specificgravity of the outer coating being substantially less than the specificgravity of the core, so that the overall specific gravity of the largerparticles is less than that of the smaller particles.

2. Particulate filter media according to claim 1, in which the specificgravity of the core material is at least twice the specific gravity ofthe outer coating material.
 3. Particulate filter media according toclaim 1, in which the outer coating material is thermoplastic resin. 4.Particulate filter media according to claim 1, in which the corematerial is silica sand.
 5. Particulate filter media according to claim1, in which the cores are of irregular shape.
 6. Particulate filtermedia according to claim 1, in which the cores have diameters in therange of about 0.005 inches to about 0.050 inches; and the overallparticle sizes are randomly distributed over the range of about 0.005inches to about 0.125 inches.
 7. A filter assembly comprising: a housinghaving an inlet at the top and an outlet at the bottom; and a bed ofparticulate filter media disposed in the housing in the line of flowfrom the inlet to the outlet; the particles of said bed defining agenerally uniform gradient of decreasing removal ratings ranging fromcoarser on top to finer on the bottom, said bed comprising amultiplicity of particles of generally decreasing size and increasingspecific gravity from top to bottom; said particles having cores ofapproximately equal size and specific gravity and outer coatings ofvarying thickness but approximately equal specific gravity, the specificgravity of the outer coating being substantially less than the specificgravity of the core, so that the overall specific gravity of the largerparticles is less than that of the smaller particles.